Growth, yield and physiological response of carrot (Daucus carota L.) to different fertilizer levels and bio-stimulants
Abstract
English: Concerns regarding environmental pollution, emanating from agricultural effluent due to
abusive use, have led to a paradigm shift in production technology. On the other hand,
subsistence farmers in developing countries cannot afford fertilizer at the current prizes.
Farmers, therefore, have to reconcile agronomic, economic, agricultural product quality
and environmental aspects of crop. In view of these concerns this study evaluated the
response of carrot (Daucus carota L. cv. Karina) to varying fertilizer levels, applied singly
and in combination with two commercial bio-stimulants, ComCat® and Kelpak®. Both biostimulants
are plant extracts, containing natural active compounds, and are applied
exogenously to manipulate crop growth and yield. No reports on the use of either of the
two products on carrot production could be found in literature. During both seasons of
greenhouse studies, the incremental increase of NPK fertilizer contributed to increased
vegetative growth and root fresh mass, though not significantly at all times. The
application of bio-stimulants had an erratic effect on carrot growth under greenhouse
conditions. However, both bio-stimulants in combination with the standard fertilizer level
enhanced root fresh mass at harvest (Chapter 3).
Under field conditions, especially during the 2007 growing season, foliar
application of both bio-stimulants enhanced yield. Although the ensuing increases were
not significant, a higher increase of 7 ton ha-1 was attained due to application of a
combination of ComCat® with the standard fertilizer. A lower increase of 4 ton ha-1 was
achieved with combinations of ComCat® and Kelpak® with the half-standard fertilizer level
(Chapter 4).
Growth and yield response of tap roots due to application of different fertilizer
levels separately and in combination with bio-stimulants was verified through
determination of selected physiological activities (Chapter 5). Sucrose content tended to
increase, in concert with yield, as the fertilizer levels were incrementally elevated.
Conversely, glucose and fructose content decreased in tap roots as maturity was attained,
probably due to less sucrose being hydrolyzed at this development stage. ComCat® had
a strong enhancing effect on sucrose translocation as evidenced by the positive
relationship between sucrose content, radio-active translocation to the roots and final
carrot root yield. The latter applied in both instances where the standard and half the standard fertilizer levels were applied in combination with ComCat®. This effect could be
related to ComCat® enabling improved sucrose transport across membranes. The effect
of Kelpak® on sucrose accumulation and translocation was less evident but, in
combination with half of the standard fertilizer, its application led to a slight increase in
yield. The accumulation of -carotene, a terpene associated with taste and quality of
carrots, was not influenced by fertilizer application. The level, however, increased where
ComCat® was combined with the half-standard fertilizer level.
To further comprehend the manipulative effects of fertilizer and bio-stimulants on
physiological processes influencing growth and yield of carrots, root respiration as well as
activities of glycolytic and oxidative pentose phosphate pathway regulatory enzymes was
determined. In all cases the activities were only measured at 30% plant development and
24 hours after second bio-stimulant application. Both ComCat® and Kelpak® increased
the respiration rate as well as the activity of glycolytic and oxidative pentose phosphate
pathway key enzymes during early carrot development under 25% and zero fertilization.
The latter was probably due to nutrient stress during early development. However, as
carrots matured nutrient acquisition and energy needs were probably reduced as
indicated by the respiration rate remaining at a steady state during later development
stages and at harvest across all fertilizer levels. This correlated positively with increased
sugar levels at maturity and the final yield.
In conclusion, the collective interpretation of growth and physiological data
acquired during this study at different fertilizer levels, especially when considered in
relationship with final yield data at harvest, does not supply a sufficient rationale to
recommend either the use of sub-optimal levels on their own or in combination with the
two bio-stimulants included in this study for the cultivation of carrots. However, it is
recommended that this study be repeated under irrigation conditions but with plots far
enough apart to prevent sideways movement of fertilizer and possible contamination of
plots. Afrikaans: Besorgdheid met betrekking tot omgewingsbesoedeling deur landbou afval as gevolg van
oormatige gebruik het aanleiding gegee tot ’n paradigmaskuif in produksietegnologie.
Aan die anderkant kan bestaansboere in ontwikkellende lande eenvoudig nie bemesting
teen die huidige pryse bekostig nie. Boere is dus verplig om agronomiese-, ekonomiese-,
produkkwaliteit- en omgewingsaspekte te versoen in hulle produksiestelsel keuse.
Hierdie studie is in die lig van hierdie besorgdheid uitgevoer ten einde die respons van
geelwortels (Daucus carota L. cv. Karina) op verskillende bemestingspeile, afsonderlik en
in kombinasie met twee kommersiële bio-stimulante ComCat® and Kelpak®, te evalueer.
Beide bio-stimulante is plantekstrakte met natuurlike aktiewe komponente wat as
blaarbespuitings toegedien word om gewasgroei en opbrengs te manipuleer. Geen
gepubliseerde verslae oor die gebruik van enige van die twee produkte op geelwortels
kon in die literatuur opgespoor word nie. Gedurende beide seisoene van glashuisstudies,
het die inkrementele verhoging van NPK bemesting aanleiding gegee tot ’n verhoging in
groeitempo en wortelvarsmassa, alhoewel nie in alle gevalle statisties betekenisvol nie.
Alhoewel blaarbespuitings met die bio-stimulante ‘n wisselvallige invloed op plantgroei in
die glashuis gehad het, het dit in kombinasie met die aanbevole standaard bemesting tot
’n verhoging in oesopbrengs onder glshuistoestande aanleiding gegee (Hoofstuk 3).
Onder veldtoestande, veral gedurende die 2007 groeiseisoen, het blaarbespuitings
met beide bio-stimulante oesopbrengs verhoog. Alhoewel hierdie verbetering nie statisties
betekenisvol was nie, is ’n oesopbrengsverhoging van 7 ton ha-1 gemeet waar ComCat® in
kombinasie met die aanbevole standaard bemesting aangewend is. ’n Laer
opbrengsverhoging van 4 ton ha-1 is gemeet waar beide ComCat® en Kelpak® in
kombinasie met die helfte van die aanbevole bemestingstandaard toegedien is (Hoofstuk
4).
Die groei- en oesopbrengsrespons van wortels op verskillende bemestingspeile,
afsonderlik en in kombinasie met bio-stimulante, is geverifieer by wyse van geselekteerde
fisiologiese aktiwiteite (Hoofstuk 5). Sukrose-inhoud het, net soos oesopbrengs, verhoog
namate die bemestingspeile in inkremente verhoog is. Die glukose- en fruktose-inhoud
het in teenstelling skerp afgeneem namate die wortels wasdom bereik het, waarskynlik
omdat minder sukrose op hierdie onwikkelingstadium gehidroliseer is. Soos aangedui deur die positiewe verwantskap tussen sukrose-inhoud, radio-aktiewe translokasie van
suiker na die wortels en die finale oesopbrengs, het blaarbespuitings met ComCat® ‘n
sterk verhogingseffek op die translokasie van sukrose gehad. Laasgenoemde is in beide
gevalle, waar die standaard en die helfte van die standaard bemesting in kombinasie met
ComCat® aangewend is, waargeneem. Hierdie invloed van ComCat® is toegeskryf aan
die vermoë daarvan om sukrose translokasie oor membrane te manipuleer. Die invloed
van Kelpak® op sukrose akkumulasie en translokasie was minder opvallend, maar in
kombinasie met die helfte van die aanbevole standaard bemestingspeil het dit tot ’n
redelike verhoging in oesopbrengs aanleiding gegee. Die akkumulasie van -karoteen, ‘n
terpeen geassosieer met smaak en kwaliteit van geelwortels, was nie verskillend by
verskillende bemestingspeile nie. Maar, waar ComCat® in kombinasie met die helfte van
die aanbevole bemestingspeil toegedien is, is die -karoteen vlak aansienlik verhoog.
Om die manipuleringseffekte van verskillende bemestingspeile en bio-stimulante
op fisiologiese prosesse wat met groei en opbrengs verband hou verder wetenskaplik te
begrond, is wortelrespirasie sowel as die aktiwiteite van glikolitiese en oksidatiewe
pentosefosfaatweg reguleringsensieme gekwantifiseer. In alle gevalle is die aktiwiteite
slegs by die 30% plantontwikkelingstadium gemeet en wel 24 uur na die tweede biostimulant
blaarbespuiting. Beide ComCat® and Kelpak® het bygedra tot ‘n verhoging in
die respirasietempo sowel as die aktiwiteite van sleutel respiratoriese en oksidatiewe
pentosefosfaat ensieme gedurende hierdie ontwikkelingstadium waar geen of 25% van
die standaard bemesting toegedien is. Laasgenoemde was waarskynlik die gevolg van ’n
stremmingstoestand wat gedurende vroeë wortelontwikkeling geskep is. Namate wortels
wasdom bereik het, het die bemesting- en energiebehoeftes van wortels waarskynlik
afgeneem soos aangedui deur die bestendige respirasietempo gedurende latere
ontwikkelingstadia en by finale oes in die geval van al vier verskillende bemestingspeile.
Laasgenoemde het positief gekorrelleer met verhoogde sukrosevlakke en oesopbrengs.
Deur hierdie studie is die aanbevole bemestingspeil vir geelwortelverbouing in
Suid-Afrika as betroubaar bevestig. Maar, waar 50% van die aanbevole standaard
bemesting in kombinasie met beide bio-stimulante toegedien is, is finale oesopbrengste
gemeet wat nie betekenisvol verskil het van dit wat met die standaard peil alleen behaal is
nie. Vanuit ’n ekonomiese perspektief is tot die gevolgtrekking gekom dat die toediening
van laasgenoemde kombinasiebehandeling vir beide bestaans- en kommersiële boere
aanbeveel kan word.
Ten slotte, die kollektiewe interpretasie van groei- en fisiologiese data wat tydens
hierdie studie met verskillende bemestingspeile bekom is, veral in die lig van finale
oesopbrengs data, verskaf nie genoegsame rasionaal om die gebruik van suboptimale
bemestingspeile, alleen of in kombinase met die twee bio-stimulante wat getoets is, vir
geelwortelverbouing aan te beveel nie. Maar, dit word aanbeveel dat hierdie studie onder
besproeiingstoestande herhaal word terwyl plotte sodanig gespasieer word dat sydelingse
beweging van bemestingstowwe nie tot kontaminasie aanleiding sal gee nie.